Project in Denmark Excavates a Basement Garage by Dredging

Danish contractor NCC Constuction used a jetting nozzle system, using dredging technology to excavate a flooded site for a below ground parking garage.

Faced with having to build a parking garage below the water table, a contractor in Copenhagen, Denmark, turned to watering and dredging as a cost-effective alternative to dewatering and excavating.

The building project in the port area of Copenhagen required a three-level, underground parking garage, and that required excavating to a depth that was several yards below sea level.

Danish contractor NCC Construction turned to the “flooded excavation principle.” After driving sheet pile into the ground to form retaining walls, the contractor flooded the site with water and pumped out the dirt with a DOP 150 submersible dredge pump from Damen Dredging Equipment in The Netherlands. The dredge pump was connected to a crane, which provided diesel power and lowered the 2400-pound pump into the water.

Dredging has been used to excavate building sites in seaside cities in Europe before, but what set this Copenhagen project apart was that NCC didn’t use a cutter head to mix soil with water to create a pumpable slurry. Instead, jetting nozzles outfitted to the end of the pump agitated the soil to create the slurry that was pumped out of the site. The jet pump used to power the nozzles was a 21,000-pound BA-C200S8 high head/high pressure pump from BBA Pumps, capable of a maximum flow of 5500 GPM and a maximum head of 445 feet. It was powered by Tier 4 Volvo Penta TAD1375VE engine.

Henno Schothorst, product manager at BBA Pumps, said NCC chose opted for jetting nozzles, because they were more efficient than a cutter head in this instance.

The jetting nozzles were powered by a jet pump, a 21,000-pound, BA-C200SB high head/high pressure pump from BBA Pumps.

The weight of the water on the site compensated for the soil being removed to stabilize the walls and the base of the excavation against artesian pressure.

Once NCC reached the intended depth, it installed underwater concrete to create a watertight base that, in combination with the retaining walls, created a caisson. After the concrete cured, NCC pumped out the water and began construction of the parking decks.